Photodynamic therapy is commonly believed to target mitochondria when porphyrin and other photosensitizers are employed. We have used multiphoton metabolic imaging of cellular NAD(P)H as to monitor the effects of Photofrin, a porphyrin photosensitizer. Both Photofrin and NADH can simultaneously be multiphoton excited with 750 nm irradiation. In both monolayer cells and multicell tumor spheroids, Photofrin is seen to cause approximately two-fold reduction of NAD(P)H autofluorescence. From observations of autofluorescence during uptake, it appears that this decrease is concurrent with Photofrin entering the plasma membrane. During PDT, NAD(P)H is observed to decrease in a dose dependent manner for relatively short irradiations. Unexpectedly, this trend eventually reverses and the signal has been observed to increase as much as 5 fold from pre-PDT levels during therapy. We are currently assessing the significance of these observations and testing hypothetical mechanisms that might explain this behavior.